Method of manufacturing a graded optical transmission medium made of synthetic resin

ABSTRACT

A method of manufacturing a multimode optical transmission medium from a synthetic resin having the distribution of refractive index varying continuously in a fixed direction. In the method, the polymerization is caused to proceed utilizing the effect of diffusion or exclusion of a monomer in the gel formed at the polymerization initiating terminal. Therefore, this method is free from the defects of methods using conventional polymerization reactions, and has an extremely high productivity.

DESCRIPTION TECHNICAL FIELD

This invention relates to a method of manufacturing a multimode opticaltransmission medium made of synthetic resin, which transmission mediumhas the optional distribution of refractive index varying continuouslyin a fixed direction. More particularly, the invention relates to amethod of manufacturing a multimode optical transmission medium havingthe distribution of refractive index varying continuously in a fixeddirection, such as optical lenses and optical fibers, in which thepolymerization is caused to proceed utilizing the effect of diffusion orexclusion of a monomer in the gel formed at the polymerizationinitiating terminal.

Background Art

As the modes of optical transmission with optical transmission mediasuch as lenses and fibers, there are two types of single mode type andmultimode type.

Among multimode type ones, the graded index type optical transmissionmedia having the distribution of refractive index varying continuouslyin a fixed direction, are widely used as bar lenses having the functionof convex lens, those having the function of concave lenses and broadband optical transmission fibers. Among them, the optical transmissionmedia made of transparent synthetic resins are more widely used inrecent years because they have several advantages in view of lightnessin weight, economy, easy handling, high impact resistance andflexibility as compared with those made of quartz.

Concerning the method of producing optical transmission media ofrefractive index distribution type which are made of synthetic resins inspecific reaction vessels, there are proposed the following methods inthe conventional art.

In Japanese Patent Publication No. 52-5857 (U.S. Pat. No. 3,955,015), amonomer to form a polymer of a different refractive index is subjectedto diffusional transfer into a specific transparent solid substance in apolymerization process having a previously formed three-dimensionalnetwork structure. After that, the whole polymerization reaction isstopped to obtain an optical transmission medium of refractive indexdistribution type.

In this method, however, it is necessary that the transparent solidsubstance is previously made into three-dimensional network structureusing a multifunctional radical polymerizable monomer, in order tomaintain the configuration of the transparent solid substance. For thisreason, it must previously be made separately, which costs much labor.In addition, the obtained polymer of three-dimensional network structureis not good in thermoplasticity and it is not suitable for drawing andother post-forming processes. In other words, the plastics-made opticaltransmission fibers should have stiffness and tensile strength as fibermaterials in the drawing step of manufacturing process. However, theoptical transmission medium prepared through the above process hasinherently three-dimensional network structure, so that it is notsuitable for drawing.

In Japanese Patent Publication No. 54-30301 and Japanese Laid-OpenPatent Publication No. 61-130904 are proposed methods for producingoptical transmission media having refractive index gradients payingattention to the difference between the monomer reactivity ratios: r₁and r₂ of two kinds of monomers.

In the above methods utilizing the difference between the monomerreactivity ratios of monomers, it is desirable that the differencebetween the radical copolymerization reactivity ratios of r₁ and r₂ islarge, as a result, the formation of homopolymer occurs at first andmacromolecules of homopolymer are formed with causing phase separation,which sometimes makes the obtained optical transmission medium cloudedto reduce the optical transmission efficiency.

When too large monomer reactivity ratios are selected, one of monomersmust be the one having a low rate of polymerization such as vinylbenzoate and vinyl o-chlorobenzoate used in the example of JapanesePatent Publication No. 54-30301, and vinyl phenylacetate used in theexample of Japanese Laid-Open Patent Publication No. 61-130904. The useof monomers whose monomer reactivity ratios are largely different meansthat monomers of considerably low reactivity are used incopolymerization. As a result, the monomer having high reactivity isfirstly polymerized and the monomer having low reactivity remains in ahigh concentration in the final stage of polymerization. Thus, it takesmuch time to complete the polymerization and, in an extreme case, theremoval of remaining monomer is required.

In addition, the existence of residual monomer causes severalundesirable influences on mechanical characteristics of transmissionmedia such as tensile strength, elongation and stiffness, and the longterm stability of transmission media owing to the postpolymerization ordecomposition of residual monomer.

The present inventors have carried out extensive investigationsconcerning the process to form copolymer resins by radical reaction.When the viscosity of monomer liquid rises and the liquid turns into gelwith the progress of the polymerization of monomers, the growing polymerradical is hardly diffused in the gel because the molecular weight ofthe polymer radical is large. In this case, the termination reactionbetween two molecules of growing polymer radicals hardly proceeds, as aresult, the rate of polymerization increases. In this state, in order topropagate the growing polymer radical further by polymerization, it isnecessary that the starting monomer is diffused in the gel and thegrowing polymer radical is continuously supplied with the startingmonomer.

The above phenomenon is accepted as the so-called gel effect in theradical polymerization. When radical polymerization is carried out sothat the gel effect is produced from any terminal point of a reactionliquid in the reaction vessel, the polymerization proceeds in a certaindirection from an optional point step by step and finally the progressof the polymerization reaches the other end portion.

The inventors of the present application have made furtherinvestigations concerning the process of connecting, for example, twokinds of different monomers to propagating polymer radicals in gel. Thatis, when a monomer becomes gel, the fractionation in a sort is caused tooccur between unreacted monomers in the gel, and one of the monomers isselectively diffused into the gel. When monomer is supplied into the gelfrom the liquid mixture, and diffused and it reaches the propagatingterminal of the polymer radical, if the rates of movement of two kindsof monomers are different in gel, only one of monomers reaches thepropagating terminal of polymer radical, as a result, the monomer havinglarger rate of movement is selectively connected to the propagatingterminal.

As a consequence, the ratio of the unreacted monomer concentration inthe gel to the monomer concentration in the liquid mixture is differentfrom one another in a plurality of monomers. It is apparent that such adifference is not owing to the monomer reactivity ratios.

In view of the above-described facts, the inventors of the presentapplication have accomplished a method of manufacturing an opticaltransmission medium made of a synthetic resin with a novel method ofcopolymerization.

The present invention can eliminate several disadvantages in the methodsof manufacturing optical transmission media of refractive indexdistribution type by the conventional polymerization process, and withutilizing the novel findings on the states of polymerization, thepresent invention attains the object to provide a method ofmanufacturing a multimode optical transmission medium having excellentcharacteristics and continuous distribution of refractive index withhigh productivity.

BRIEF DESCRIPTION OF DRAWINGS

Each of FIG. 1 and FIG. 2 shows distribution of refractive index in thedirection of radius of an optical fiber, which is prepared in Example 1and Example 2, respectively. In both the figures, the axis of ordinateindicates the differences (Δn) between the highest refractive index andrefractive indexes at specific distances.

DISCLOSURE OF INVENTION

The present invention relates to a method of manufacturing aplastics-made graded index type optical transmission medium having thedistribution of refractive index varying continuously in the directionof the progress of polymerization, which is attained by lowering theratio of monomer having a higher ratio of the unreacted monomerconcentration in the gel to the monomer concentration in the liquidmixture, gradually in the direction of the progress of polymerization,and characterized in the use of monomers in which the ratio of unreactedmonomer concentration in the gel to the monomer concentration in theliquid mixture of one monomer is different from those of other monomersand the difference in refractive indexes of homopolymers of the monomersis at least 0.005, in the radical polymerization from an optionalposition in a vessel containing a liquid mixture of a plurality ofradical copolymerizable monomers to polymerize the monomers through gelcondition into a polymer.

A second characteristic point according to the present invention is thatthe monomer reactivity ratios of any of the foregoing radicalcopolymerizable monomers are not less than 0.2, preferably more than0.5.

A third characteristic point according to the present invention is thatthe polymerization is caused to proceed from the wall surface of areaction vessel to the inner part of the vessel.

The present invention will be described in more detail.

In the present invention, a liquid mixture of a plurality of monomers isfed into a vessel, which vessel may be in any configuration such ascolumn, square plate or sphere, preferably column, and more preferablycylinder. The radical copolymerization is then caused to proceed fromany terminal portion such as a part of wall surface of the vessel toother part such as an inner part of the vessel under the gel effect. Thesizes of the vessel are not limited, that is, a vessel of optional sizescan be employed. For example, when the vessel is a cylindrical one, itsinner diameter is suitably from 1 to 70 mm.

In the polymerization, it is possible to use a solvent. In the case thata solvent is used, the removal of the solvent is necessary after thepolymerization and there occurs some ill effect in the removal of thesolvent, therefore, it is generally advisable that the polymerization isdone using the monomer itself as a solvent without using any othersolvent.

In the first place, energy to produce radicals such as heat orultraviolet rays is locally applied from the side of vessel wall by anappropriate known means so as to produce a portion of high temperatureor a portion of intense ultraviolet rays in the monomer liquid mixturenear the vessel wall, thereby forming radicals of high concentration insuch portions and the polymerization is caused to proceedpreferentially. It is possible in the case of a cylindrical or aspherical vessel having a rotary axis that the vessel is rotated on theaxis at 1000 rpm or less. However, mechanical movement such as rotation,stirring or vibration to destroy or disturb the gel condition is notdesirable.

The wavelength and the temperature of heating for the radicalpolymerization can be optionally selected in accordance with the kindsof monomers. For example, the heating temperature is in the range ofroom temperature to 150° C. In any case, a radical polymerizationinitiator such as benzoyl peroxide (BPO) or a photopolymerizationsensitizer can be used as occasion demands. Both the photopolymerizationand the thermal polymerization can be caused to occur simultaneously.

When the radical polymerization proceeds to increase the viscosity ofmonomer liquid mixture into a gel condition, the polymer radical ishardly diffused in the gel and the probability to terminate the reactionin the polymerization is low. As a result, the rate of polymerization inthe gel portion is raised. The radical propagation terminals in the gelfurther connect with unreacted monomers to proceed the polymerizationand a resin is finally produced. At the same time, the gel ispolymerized successively in the direction of polymerization of the frontface of polymerized resin. It is, thus, possible to forward successivelythe polymerization in the direction from the wall of the vessel to theinside of the vessel with utilizing the gel effect. The polymerizationinitiating terminal may be selected optionally from any point such asthe inside wall of a vessel and an inner part of a vessel. Thepolymerization is started generally from a wall surface of a vesselbecause heat or light rays can be readily applied.

The term "gel" herein referred to means an oligomer or polymer in whichthe viscosity is raised to such a degree that the polymer propagatingradical cannot substantially be diffused. The oligomer or polymer may becomposed of any one of the monomers in the monomer mixture or may becomposed of a plurality of monomers. In some cases, the produced gel isseparated out from the monomer mixture. However, the polymer in whichthe polymerization degree is so high that the migration of monomer isimpossible in the produced gel, is not included. By the way, too large arate of polymerization is not desirable because the polymerization iscompleted without forming a distinct gel condition. In view of this, therate of polymerization may be determined properly so that the monomercan be sufficiently migrated in the gel. The time of polymerization isgenerally selected from the range of 1 to 100 hours.

Sufficient gel effect can be produced by setting the gel on the insidewall of a vessel in the beginning of the polymerization initiation step.For this purpose, it is desirable that a vessel made of a material whichhas large affinity with the monomer to be polymerized. For example, thedesirably used vessel is made of a polymer which is made from the samemonomer as the used monomer that is more diffusive into the gel or whichis made from a monomer having large affinity with such a monomer.

In the present invention, the monomer in which the ratio of theunreacted monomer concentration in the gel to the monomer concentrationin the liquid mixture is substantially different, must be used. Thisunreacted monomer concentration in the gel means the concentration ofmonomer in the unreacted monomer mixture which remains near the polymerpropagation terminal in the gel. Furthermore, the monomer concentrationin the liquid mixture means the concentration of monomer in the monomerliquid mixture in contact with the above gel.

Provided that the unreacted monomer concentrations in a gel and themonomer concentrations in the liquid mixture are M₁ ^(g), M₂ ^(g), M₁^(l), M₂ ^(l) in the case of two kinds of monomers M₁ and M₂, thedefinition that the ratios of unreacted monomer concentration in the gelto the monomer concentration in the liquid mixture are different, meansthe relationship represented by the following equation: ##EQU1##

In the mixture of monomers in which the above ratios of concentrationare the same, the ratio of monomer composition in the gel is the same asthe ratio of monomer composition in the liquid mixture. In the systemlike this, the ratio of monomer composition contained in the polymer tobe produced, is determined only by the ratio of reactivities ofmonomers, which is not desirable because the foregoing defects arecaused to occur. In other words, the monomer having higher reactivity isfirstly polymerized and the monomer having lower reactivity remains in ahigh concentration in the final stage of polymerization. It is notdesirable because it takes much time to complete the polymerizationowing to the polymerization of the remaining monomer and, in addition,in an extreme case, the remaining monomer must be removed.

In the present invention as described in the foregoing passage,fractionation effect is caused to occur in the unreacted monomerexisting in the gel by the formation of gel, as a result, partiality ofspecific monomer is caused to occur. Monomers move from the monomermixture to the inner part of the gel, however, the velocities aredifferent among monomers. As a result, the ratio of monomer compositionin the monomer mixture is different from the ratio of monomer(unreacted) composition in the gel.

Although the polymer propagation radical is not diffusive substantiallyin the gel which is formed in the polymerization, it is necessary thatall the above monomers have sufficient diffusion velocity. This is forthe reason that, if a monomer is not fed from the monomer mixture whichis in contact with gel to the propagation terminal of polymer radical,the polymerization cannot proceed.

The monomers used in the present invention are those in which theforegoing ratios of the unreacted monomer concentration in gel to themonomer concentration in liquid mixture are substantially different fromone another. In addition, it is necessary that the refractive indexes ofthe homopolymers, which are previously determined after polymerizingseparately, are different. In other words, the difference in therefractive indexes of homopolymers of monomers is at least 0.005. Byusing the above monomers, in the ratio of monomer composition in thepolymer to be obtained, the content of monomer is decreased in thedirection of the progress of polymerization, which monomer is higher inthe ratio of the unreacted monomer concentration in gel to the monomerconcentration in liquid mixture, thereby obtaining a synthetic resinoptical transmission medium having the refractive index gradation in thedirection of the progress of polymerization. When monomer mixture inwhich the difference of refractive indexes of homopolymers is smallerthan 0.005 is used, the gradation in refractive index cannot be attainedeven if the ratio of monomer composition of the obtained polymer isgradient.

The monomer according to the present invention can be determined bymeasuring the unreacted monomer concentration in gel and the monomerconcentration in liquid mixture by an optional method. The measurementof monomer concentration in gel is, however, practically difficult.Accordingly, in order to facilitate the selection of monomers inpractice, it is desirable to use the following standards of intrinsicvolumes of monomers or solubility parameters of monomers.

(1) Intrinsic Volumes of Monomers

The movement of monomers in gel relates to the intrinsic volumes ofmonomer. Provided that the intrinsic volumes of two kinds of monomers M₁and M₂ in a monomer mixture are V₁ and V₂, respectively, two kinds ofmonomers which meet the following equation (1) are selected. ##EQU2##

The intrinsic volume of monomers can be calculated with the van derWaals radii and inter-atomic distances [e.g., method of G. L. Slonimskiiet al. (cf: Vysokomol. soyed. A12: No. 3, 494-512, 1970)].

(2) Solubility Parameters of Monomers

The solubility parameter of monomer also relates to the moving velocityin gel. Accordingly, provided that the solubility parameters of twokinds of monomers M₁ and M₂ in a monomer mixture are δ₁ and δ₂,respectively, and the solubility parameter of polymer forming a gel isδ_(p), two kinds of monomers which meet the following equation (2) areselected. ##EQU3##

In the above equation, the solubility parameters of monomers and polymercan be calculated by the following equation [cf: Method of Hoy et al.(POLYMER HANDBOOK, Third edition, VII/519, published by WileyInterscience)]. ##EQU4##

In the above equation, d and M are respectively density and molecularweight of monomer or polymer. G is Group Molar Attraction Constant.

Any of the combination of monomers which does not meet the aboveequations (1) or (2) is not desirable for the above-described reasonbecause the ratios of the monomer concentration in gel to the monomerconcentration in liquid mixture substantially coincide with each other.

For reference purpose, monomer intrinsic volumes and solubilityparameters of some radical polymerizable monomers are shown in thefollowing Table 1.

                                      TABLE 1                                     __________________________________________________________________________               Intrinsic         Solubility                                                  Volume of                                                                           Refractive                                                                          Molecular                                                                           Parameter                                                   Monomer                                                                             Index of                                                                            Weight of                                                                           (cal/cm.sup.3).sup.1/2                           Monomer    (Å.sup.3)                                                                       Polymer                                                                             Monomer                                                                             Polymer                                                                            Monomer                                     __________________________________________________________________________    4-Methylcyclohexyl-                                                                      185.0 1.4975                                                                              182.3 9.16 --                                          methacrylate                                                                  Cyclohexyl-                                                                              177.6 1.5066                                                                              168.2 9.04 8.25                                        methacrylate                                                                  Furfurylmethacrylate                                                                     159.6 1.5381                                                                              166.2 9.93 --                                          1-Phenylethyl-                                                                           197.0 1.5487                                                                              190.2 9.29 8.52                                        methacrylate                                                                  1-Phenylcyclohexyl-                                                                      263.0 1.5645                                                                              244.3 8.91 --                                          methacrylate                                                                  Benzylmethacrylate                                                                       180.0 1.5680                                                                              176.2 9.54 8.87                                        1,2-Diphenylethyl-                                                                       272.8 1.5816                                                                              266.3 9.49 --                                          methacrylate                                                                  o-Chlorobenzyl-                                                                          194.7 1.5823                                                                              210.6 9.71 --                                          methacrylate                                                                  p-Chlorobenzyl-                                                                          194.7 1.5823                                                                              210.6 9.71 9.21                                        methacrylate                                                                  Diphenylmethyl-                                                                          255.7 1.5933                                                                              252.3 9.54 --                                          methacrylate                                                                  Pentachlorophenyl-                                                                       236.6 1.608 344.4 10.40                                                                              --                                          methacrylate                                                                  Pentabromophenyl-                                                                        270.5 1.71  556.7 10.02                                                                              --                                          methacrylate                                                                  Methylmethacrylate                                                                       104.4 1.492 100.1 9.20 7.82                                        Styrene    117.8 1.591 104.0 9.28 8.15                                        Phenylmethacrylate                                                                       163.0 1.5706                                                                              162.2 9.65 9.13                                        __________________________________________________________________________

The radical polymerizable monomers used in the present invention aremonofunctional monomers having one functional group which is active inradical polymerization such as the double bond of allyl group, vinylgroup, acrylic group, and methacrylic group. The polyfunctional monomerswhich form three-dimensional network polymer are not included. However,a small quantity of these polyfunctional monomers can be added withoutdeparting from the scope of the present invention.

Furthermore, it is defined as desirable conditions in the presentinvention that the ratio r of monomer reactivity ratio is not smallerthan 0.2 and preferably more than 0.5. The monomer reactivity ratios r₁and r₁ are respectively the ratios of coefficients of polymerizationrate k₁₁ /k₁₂ and k₂₂ /k₂₁ in copolymerization of two kinds of monomers(M₁ and M₂) which are represented by the following equations. ##EQU5##In the above equations, the symbols [M₁.], [M₂.], [M₁ ] and [M₂ ]indicate respectively the concentrations of polymer propagation radicalM₁., polymer propagation radical M₂., monomer M₁ and monomer M₂.

In the case of two kinds of monomers, the monomer reactivity ratios aretwo of r₁ and r₂. When 3 kinds of monomers are used, the ratios increaseto 6 kinds. Also in the case of 3 kinds of monomers, the 6 reactivityratios are all not less than 0.2, preferably more than 0.5.

Even in the case of monomers which are substantially different in theratios of unreacted monomer concentration in gel to monomerconcentration in liquid mixture, if the monomer reactivity ratios do notcome within the above range, the monomers are hardly copolymerized toeach other, so that a more reactive monomer is firstly polymerized andthe monomer composition ratio in the polymer is biased to one monomer.In an extreme case, the transparent polymer becomes cloudy by theformation of homopolymer, or less polymerizable monomer remainsunreacted in the final stage of the polymerization.

The monomers used in the present invention is required only to meet theabove conditions and there is no other limitation. It is possible toselect optionally two or more of monomers from those listed in theforegoing Table 1. Among them, the combination of benzylmethacrylate andmethylmethacrylate is especially desirable because the monomers arereadily available and in view of the transparency of obtained opticaltransmission media and the good drawing property of obtained polymer.

As described in the following examples, when a liquid mixture ofbenzylmethacrylate and methylmethacrylate is polymerized from the wallportion of a cylindrical vessel, benzylmethacrylate is denselydistributed in the middle portion and methylmethacrylate is distributednear the inside wall surface in the obtained resin. That is, a convextype optical transmission medium in which the refractive index in thecentral portion is higher than that of the inside wall portion.

However, because the monomer reactivity ratios of methylmethacrylate andbenzylmethacrylate are 0.93 and 1.05, respectively, according to thedescription in Japanese Patent Publication No. 54-30301 and JapaneseLaid-Open Patent Publication No. 61-130904, it is considered that thebenzylmethacrylate having the higher reactivity ratio is biased to theinside wall of the vessel and methylmethacrylate having the lowerreactivity ratio is biased to the central portion to produce a concavetype optical transmission medium in which the refractive index in thecentral portion is smaller than that of the inside wall portion.According to the experiments described below, however, exactly oppositeresults are obtained.

The above description referred to the case in which two kinds ofmonomers are used. The number of monomers, however, is not limited totwo but more than two kinds of monomers can be used. When three kinds ofmonomers are used, each of them must meet the above conditions.

As far as no ill effect is produced in the transparency of the productpolymer, optional additives such as radical polymerization initiator,antioxidant or else can be added.

For example, when the polymerization is caused to proceed in the radialdirection of a cylindrical vessel, a transparent resin rod havingradially graded refractive index can be obtained. This rod can be usedas an optical transmission medium as it stands or by applying properprocessing. For example, the obtained rod is drawn at an optionaldrawing ratio by a known method into fibers to produce opticaltransmission fibers made of a synthetic resin.

A cylindrical transmission medium having refractive index distributionof convex type, where the central portion is high in refractive index,is used for a bar lens and optical fiber for optical communicationhaving the function of convex lens. A cylindrical transmission mediumhaving refractive index distribution of concave type, where the centralportion is low in refractive index, is used for a bar lens and opticaltransmission media having the function of concave lens.

When the reaction is carried out in a rectangular vessel, a plate lenshaving the function of convex lens or concave lens, is produced.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be described in more detail with reference toexamples.

EXAMPLE 1

Methylmethacrylate (MMA) was put into a horizontally held glass tube andboth ends of the tube were sealed up. Thermal polymerization was donewith rotating the tube at 1000 rpm according to an ordinary method. Apolymer tube of 10 mm in outer diameter and 6 mm in inner diametercomposed of polymethylmethacrylate (PMMA) of 100,000 in molecular weightwas prepared.

The outer glass tube was cracked and removed. MMA and benzylmethacrylate(BzMA) in the ratio by weight of 4:1 were filled into the horizontallyheld PMMA tube. After mercaptan and 0.50 wt. % of a polymerizationinitiating agent of benzoyl peroxide (BPO), thermal polymerization wasdone at 70° C. for 20 hours in the atmosphere. During thepolymerization, the polymer tube was rotated at 1000 rpm.

After the polymerization, it was subjected to thermal treatment under areduced pressure of 0.2 mm Hg at 80° C. for 20 hours. The content ofmonomer remaining in the produced polymer was measured, which was lessthan 0.5 wt. %.

The polymer tube and the polymer in the tube were integrally combined.Both end portions of this were cut off and it was thermally drawn with acylindrical heating tube by indirect heating at 250° C. to obtain anoptical fiber of 0.6 mm in diameter.

The distribution of refractive index in radial direction of the obtainedoptical fiber was measured by lateral interference method. The resultantdata of distribution is shown in FIG. 1, which was even throughout thewhole length of the fiber.

EXAMPLE 2

MMA and BzMA in a weight ratio of 4:1 were fed into a horizontally heldglass tube and both ends of the tube were sealed up. Thermalpolymerization was done with rotating the tube at 1000 rpm according toan ordinary method. A polymer tube of 10 mm in outer diameter and 6 mmin inner diameter composed of MMA/BzMA copolymer of 100,000 in molecularweight was prepared.

The outer glass tube was cracked and removed. MMA and BzMA in a weightratio of 4:1 were filled into a horizontally held copolymer tube.Polymerization was carried out in the like manner as in Example 1 withrotation. The content of monomer remaining in the produced polymer wasless than 0.5 wt. %.

By treating in the like manner as in Example 1, an optical fiber of 0.6mm in diameter was obtained.

The distribution of refractive index in radial direction of the obtainedoptical fiber was measured by lateral interference method. The resultantdistribution is shown in FIG. 2 which was even throughout the wholelength of the fiber.

EXAMPLE 3

An optical fiber of 0.6 mm in diameter was made in the like manner as inExample 1 except that phenylmethacrylate (PhMA) was used in place ofBzMA.

The measurement was done with regard to the obtained optical fiber, as aresult, it was understood that the distribution of refractive index wassimilar to that shown in FIG. 1.

EXAMPLE 4

MMA and BzMA in weight ratios of 3:1, 4:1 and 5:1 were fed into threePMMA tubes obtained in the like manner as in Example 1, respectively.After adding 0.15 wt. % of a chain transfer agent of n-butylmercaptanand 0.50 wt. % of a polymerization initiating agent of an organicperoxide (trademark: PERHEXA 3M (made by Nippon Oils And Fats Co.,Ltd.)), thermal polymerization was done at 90° C. for 20 hours in theatmosphere. During the polymerization, the polymer tube was rotated at1000 rpm.

After that, optical fibers of 0.6 mm in diameter, respectively, wereobtained by thermal drawing in the like manner as in Example 1.

The distributions of refractive indexes of the three optical fibers weremeasured. The differences of refractive indexes between central portionsand peripheral portions were 0.017, 0.014 and 0.012, respectively, atmaximum and the refractive indexes varied continuously.

EXAMPLE 5

Optical fibers of 0.6 mm in diameter were made in the like manner as inExample 1 except that p-chlorobenzylmethacrylate (CBzMA),1-naphthylmethacrylate (NMA) and p-bromobenzylmethacrylate (BBzMA) wereused in place of BzMA. The measurement was done with regard to theobtained optical fibers, as a result, it was understood that all had thedistribution of refractive indexes of convex type similar to that ofFIG. 1.

EXAMPLE 6

An optical fiber of 0.6 mm in diameter was made in the like manner as inExample 2 except that PhMA was used in place of BzMA. The measurementwas done with regard to the obtained optical fiber, as a result, it wasunderstood that the distribution of refractive index was similar to thatshown in FIG. 2.

INDUSTRIAL APPLICABILITY

According to the present invention, the defects of phase separation andclouding due to the formation of homopolymer, the problem of remainingmonomers due to the large difference in polymerization rates and thenecessity of a long time for the completion of reaction in the use ofmonomers which are not good in monomer copolymerizability in theconventional art, were eliminated. Thus a large improvement was obtainedin that a desirable optical transmission medium having refractive indexgradient of multimode graded index (GI) type such as optical fiber andoptical lens, is produced.

Furthermore, because the material is made of a thermoplastic resin, thedrawing as post-forming process is possible, thereby readily providingfibers of a desired configuration.

As described in the above examples, when a liquid mixture ofbenzylmethacrylate and methylmethacrylate or phenylmethacrylate andmethylmethacrylate is polymerized from the side of wall surface of acylindrical vessel, benzylmethacrylate or phenylmethacrylate is denselydistributed in the central portion and methylmethacrylate is denselydistributed on the side of inner wall and a convex type opticaltransmission medium can be obtained, in which the refractive index inthe central portion is higher than that in the portion near the insidewall.

The monomer reactivity ratios r of methylmethacrylate andbenzylmethacrylate are 0.93 and 1.05, respectively. These values r ofmethylmethacrylate and phenylmethacrylate are 0.56 and 1.72,respectively. Therefore, in accordance with the methods disclosed in theforegoing Japanese Patent Publication No. 54-30301 and JapaneseLaid-Open Patent Publication No. 61-130904, it is considered thatbenzylmethacrylate or phenylmethacrylate is densely distributed on theside of inner wall and methylmethacrylate is densely distributed in thecentral portion to produce a concave type optical transmission medium inwhich the refractive index in the central portion is smaller than thatof the inner wall portion. Therefore, this means that the presentinvention cannot be made by the analogy of the disclosure in the abovepatent gazettes.

We claim:
 1. A method of manufacturing a plastics-made graded index typeoptical transmission medium having a distribution of refractive indexvarying continuously in the direction of progress of polymerizationcomprising the steps of:feeding at least two radically copolymerizablemonomers into a reaction vessel whereby a liquid mixture is formed, saidmonomers separately polymerizable into homopolymers having refractiveindexes which differ from each other by at least 0.005; subjecting saidmonomers to radical polymerization initiated at an optional position insaid reaction vessel whereby said monomers are polymerized through a gelcondition into a polymer; polymerizing said monomers wherein the ratioof the concentration of unreacted monomer in the gel to theconcentration of unreacted monomer in the liquid reaction mixturediffers from the corresponding ratios of the other monomers fed into thereaction vessel; and lowering the monomeric concentration of the monomerhaving the highest ratio of unreacted monomer concentration in the gelto unreacted monomer concentration in the liquid mixture, in thedirection of progress of polymerization.
 2. The method as claimed inclaim 1, wherein the monomer reactivity ratio r of any of said radicalcopolymerizable monomers is not lower than 0.2.
 3. The method as claimedin claim 2, wherein the monomer reactivity ratio r of any of saidradical copolymerizable monomers is not lower than 0.5.
 4. The method asclaimed in claim 2, wherein said polymerization is caused to proceedfrom the wall surface of the vessel to the inside of the vessel.
 5. Themethod as claimed in claim 1, wherein said monomer liquid mixturecontains at least methylmethacrylate and benzylmethacrylate.